It is known in the art to provide a gyro sensor using a piezoelectric crystal oscillator for use as a physical quantity sensor for attitude control of a car navigation system, robot, etc. (for example, refer to patent document 1).
FIG. 1 is a diagram showing one example of a prior art gyro sensor.
As shown in FIG. 1, gyro sensor 1 comprises an oscillation circuit 3, which includes a crystal oscillator 2 having detection electrodes 5 and 6, and a detection circuit 10 for detecting a Coriolis force based on detection signals supplied from detection electrodes 5 and 6. Detection electrodes 5 and 6 are formed on a detection tine of crystal oscillator 2 and, based on the outputs from the driving electrodes formed on a driving tine of crystal oscillator 2, oscillation circuit 3 performs binarization and outputs a detection clock CL in the form of a rectangular wave.
Crystal oscillator 2 continues to oscillate with a constant amplitude under the control of oscillation circuit 3; if, at this time, crystal oscillator 2 is rotated with an angular velocity ω, a Coriolis force F proportional to the angular velocity ω acts at right angles to the direction of vibration of the driving tine of crystal oscillator 2. Then, due to the stress induced by the Coriolis force F, crystal oscillator 2 is set into vibration at a frequency equal to the drive frequency, as a result of which electrical charges due to the piezoelectric effect are set up on detection electrodes 5 and 6 formed on the detection tine.
These charges cause detection currents I1 and I2, very small currents of opposite phases, to flow in detection electrodes 5 and 6, respectively. I/V conversion circuits 11 and 12 in detection circuit 10 convert detection currents I1 and I2 into detection voltages V10 and V11, respectively, and a differential amplifier 13 amplifies the difference between detection voltages V10 and V11, and thus produces a difference output V12. A synchronous detection circuit 14 takes difference output V12 as input, performs synchronous detection by synchronizing the timing with the detection clock CL output as a rectangular wave from oscillation circuit 3, and produces a detection output V13. A low-pass filter (LPF) 15 cuts off the AC component of detection output V13, and outputs an angular velocity detection signal V14 which is a DC voltage proportional to the angular velocity.
FIG. 2 is a diagram showing signal examples in synchronous detection circuit 14.
FIG. 2(a) shows the case where the difference output V12 of the differential amplifier 13 is input to the synchronous detection circuit 14, FIG. 2(b) shows the case where noise 1 at twice the frequency of difference output V12 is input to synchronous detection circuit 14, and FIG. 2(c) shows the case where noise 2 at three times the frequency of difference output V12 is input to synchronous detection circuit 14.
As shown in FIG. 2(a), difference output V12 is detected with the detection clock CL to produce detection output V13 whose AC component is then cut off by LPF 15, producing the angular velocity detection signal V14, which is a DC voltage having a certain value.
As shown in FIG. 2(b), when noise 1 at twice the frequency of the difference output V12 is input to synchronous detection circuit 14, noise 1 is detected by the detection clock CL, but since synchronous detection output V13 in this case has an upper-lower symmetrical waveform, the output that LPF 15 produces by cutting off the AC becomes zero, hence no ill effect on the angular velocity detection signal V14. On the other hand, as shown in FIG. 2(c), when noise 2 at three times the frequency of difference output V12 is input to synchronous detection circuit 14 and detected by detection clock CL, resulting synchronous detection output V13 has an upper-lower asymmetrical waveform; as a result, even if the AC is cut off by the LPF 15, the DC component, and hence noise, remains in angular velocity detection signal V14. While FIG. 2(c) has been described for the case where noise at three times the difference output V12 is input, the same problem occurs when harmonic noise at an odd multiple of the frequency of the difference output V12 is input.
That is, there has been the problem that when harmonic noise superimposed on the detection signal is input to the synchronous detection circuit, the angular velocity detection signal is affected by the noise.
Patent document: Japanese Unexamined Patent Publication No. 2007-57340 (FIG. 9)